The production of recycled fiber has become an important element of the pulp and paper business. Advances in the technology of printing as well as the broadening of the scope of recyclable printed material present new challenges for recyclers. Thus, innovations in the deinking process must address the problems created by novel inks and print media. For example, the water-based inks which have evolved for use in flexographic newspaper: printing present difficulties for recycling in that residual ink of this nature which passes through the deinking process causes a reduction in the brightness of recycled paper.
Conventional deinking (such as that used for newsprint deinking) is comprised of three distinct steps. First the ink is removed, or separated, from the paper fiber. This usually occurs while the waste paper is being repulped in water and is routinely aided by caustic and surfactant. Secondly, the separated ink is dispersed to a small particle size. The dispersion step usually occurs concomitantly with the separation step. In the third step, the dispersed ink is usually separated from the repulped fiber slurry by washing or flotation.
Efficient deinking demands both a successful separation of ink from the fiber and a successful separation of the ink dispersion from the fiber slurry. A deinking process that successfully separates the ink from the fiber and successfully disperses the ink into the aqueous phase of the slurry as small particles will be inadequate if thereafter it does not provide effective removal of the ink from the fiber slurry. Dispersed ink particles that are carried along with the fiber in the slurry will in some degree be retained on the fiber during paper formation, resulting in a general gray hue or distinct spots, and commonly a low brightness.
Large volumes of water are required for the ink removal process. Clean-up of the ink laden process water is typically accomplished by a solid/liquid separation unit operation, such as Dissolved Air Flotation (DAF). Since recycle mills are frequently located in metropolitan areas, a great emphasis is placed on maintaining a closed water cycle in the mill. Effective clean-up of the DAF influent is important, for residual ink remaining in the water may be reintroduced to the fibers, resulting in a loss of brightness to the final sheet.
Clarification chemicals are typically used in conjunction with DAF units to remove the suspended solids from the deinking process waters. Often, deinking process waters have a large anionic character which requires the use of a dual polymer program for effective clean-up. A dual polymer treatment generally comprises a low molecular weight cationic coagulant in combination with a high molecular weight flocculant. Typical cationic coagulants are poly(diallyldimethylammoniumchloride), amphoteric diallyldimethylammoniumchloride containing copolymers, condensation polymers of ethylene dichloride/ammonia or dimethylamine/epichlorohydrin. Acrylamide-based flocculants have been utilized to assist in the solid/liquid separation. Both anionic and cationic flocculants can be employed.
It is common to coagulate the suspended solids contained in the deinking process waters by using as a coagulant a water soluble cationic polymer. One of the polymers that has met with some success is polydiallyldimethylammoniumchloride, hereafter poly(DADMAC). One of the drawbacks in using this coagulant is that it is relatively expensive and the dosages necessary to effectuate good coagulation are often excessive. If it were possible to provide a more effective coagulant other than poly(DADMAC), an improved clarification of waste deinking process waters would be afforded.
Noteworthy attempts which have improved the efficiency of diallyldimethylammoniumchloride as a deinking agent are disclosed in U.S. Pat. Nos. 4,715,962, 5,013,456 and 5,207,924. These patents show that certain amphoteric copolymers of diallyldimethylammoniumchloride with anionic monomers, such as acrylic acid, improve efficiency of poly(DADMAC) as a deinking water coagulant.